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MICROVERTEBRATE CONCENTRATIONS IN PEDOGENIC NODULECONGLOMERATES: RECOGNIZING THE ROCKS AND RECOVERINGAND INTERPRETING THE FOSSILS

J. A. Schiebout, Suyin Ting, and J. T. Sankey

ABSTRACT

Pedogenic carbonate nodule concentrations are an under-utilized resource for therecovery of terrestrial microvertebrate faunas. They have proved productive in LateCretaceous, late Paleocene, and Miocene rocks of Texas and Louisiana. Theseconcentrations of nodules and fossils form at channel bases and by erosion of soils ininterfluves. Treatment of large samples of the conglomerates with dilute acetic acidallows recovery of delicate specimens, with teeth of small vertebrates being the mainidentifiable component. Geologists and paleontologists should be alert to these rocks’potential, because relatively small lenses, which can be found in gullies and streambanks even in heavily vegetated areas such as western Louisiana, may yield significantfossil faunas.

J.A. Schiebout and Suyin Ting, Museum of Natural Science, Louisiana State University, BatonRouge, LA 70803, USA. Correspondence should be directed to Schiebout.J.T. Sankey, Department of Geology and Geophysics, Louisiana State University, Baton Rouge,LA 70803, USA

Keywords: microvertebrates, conglomerates, acid treatment, taphonomy, pedogenic nodules

Submission: 20 September 1997Acceptance: 28 July 1998

Copyright: Paleontological Society, 1 August 1998http://palaeo-electronica.org/1998_2/schiebout/issue2.htm

INTRODUCTION

Concentrations of pedogenic nodules do not look promising as fossil sources, but atleast in the US Gulf Coast, have proved very productive. They have been especiallyvaluable in a recent large-scale project involving the first sizable Miocene vertebratefaunas from Louisiana. We have explored a variety of methods for efficiently extractingdelicate teeth (Figure 1), often smaller than one mm in crown maximum diameter, fromrocks that look a lot like concrete curbing, and have developed methods suitable tosmall lab spaces and low manpower.

Earlier work predisposed Schiebout in 1993 to recognize a layer of conglomeratecontaining pedogenic nodules, in a manmade exposure at Fort Polk in westernLouisiana (Figure 2), as a significant paleontological opportunity. She had been calledto the site to examine a recently found, nodule-encrusted merychippine horse partialmandible (Figure 3). The layer of nodule-rich conglomerate at Fort Polk was the first ofseven sites which have yielded over 3,000 catalogued fossils since 1993. Schiebout’s

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work on such rocks in Big Bend National Park, Texas, prepared her to recognize otherfossil-rich conglomerates. Fossil hunting had been ongoing in the Paleocene BlackPeaks Formation in Big Bend for more than twenty years, but had yielded fewvertebrates in the small size ranges when she began work in 1968. Small mammal teetheroding from a lens of pedogenic nodules, when screened, added a small-mammalcomponent to the late Paleocene of Big Bend (Figure 2 and Figure 4; Schiebout 1974,1979, 1981; Schiebout et al. 1987; Schiebout et al. in press). Later surveying forscreening sites in the Late Cretaceous to early Eocene of Big Bend in the 1980srevealed a layer of nodule-rich conglomerate in the Late Cretaceous Aguja Formation(Figure 5). Many shark, fish, lizard, dinosaur, and mammal teeth have been recoveredthrough dissolution and screening from it and other Aguja Formation conglomeratehorizons in its vicinity (Sankey 1995, 1996, 1997; Sankey and Schiebout 1997, Sankey1998).

MATERIALS AND METHODS

Claude Hibbard (1949) standardized the process of bulk screening for vertebrate fossilsfrom rocks that disaggregate in water by developing wooden boxes with screen bottomsand two partially screened sides, which could be placed in a stream and agitated toremove the mud (Figure 6). Acid treatment to remove calcium carbonate from non-microscopic vertebrate fossils is discussed in Rutsky et al. (1994). The unusual elementof our procedures is the large-scale bulk screening of rocks that must be partiallydissolved first.

Collecting in the field is more similar to micropaleontological sampling in which sedimentsamples are bagged for later treatment than to most methods of vertebratepaleontological collecting, except that the bags are much larger and several may berequired even for a test sample of a site. We have used burlap bags (Figure 7 andFigure 8) and currently use woven plastic "burlap" bags and close them with duct tape(Figure 9). Some of the conglomerates are the hardest rocks in the area and requireusing an airless jackhammer and sledge hammer to detach pieces. In some of thesecases, for example at Stonehenge Site (Figure 10) in the Fort Polk Miocene, a gasoline-powered jackhammer has been used to break up the rock for transport.

Current laboratory treatment of the Fort Polk Miocene conglomerates involves soakingpieces, usually between three and forty cm in diameter, in approximately 10% aceticacid to partially dissolve cement, releasing the fossils and nodules as a residue (Movie1). Calcium carbonate, which is dissolved in weak acetic acid, is the main mineral in thenodules and in the cement holding the rock together. Teeth and bones are composed ofapatite and are not dissolved. Sankey and students under her direction haveexperimented upon Cretaceous conglomerate from Big Bend, Texas, and concludedthat up to 40% acetic acid can be used without harm to vertebrate teeth from theseconglomerates, but a 10% concentration has been seen to cause erosion of dentine in afew instances on the Fort Polk Miocene specimens, so the largest of our dissolutionoperations continues at that level of acid concentration.

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Bulk acid lab procedure on the Louisiana Miocene rocks has undergone a process ofrefinement since 1993 as efforts to dissolve tons of rock in a cost- and time-efficientmanner continue in small lab spaces with relatively little manpower. Heavy plastic 10-gallon lidded storage boxes (Rubbermaid Rough Totes©) were used in an initial stage,which was supplanted by a method utilizing 85-gallon barrels (hazardous wasteoverbarrels) containing nets in frames to hold the rocks suspended in acid solution,which were lifted and handled with a hoist on wheels (Dooley et al. in press). Thisprocedure is worth considering in some circumstances, but we have returned to usinglarge numbers of 10-gallon lidded boxes, which are easier to handle for small toaverage-sized adults. They also offer a simple way to isolate and test small (less than50 kg.) initial samples, and use of them allows having as much rock in acid in anequivalent square footage as the large barrels do.

Currently, two people work four hours a week in the acid lab, handling 53 dissolvingboxes. Approximately 10 kg portions of conglomerate are initially placed into each boxand covered with 10% acetic acid. The amount disaggregated from Fort Polk last yearwith this method is 1,049 kg. Boxes are screened on a three-week cycle of rotation, withsixteen usually being washed each week. They are moved on a trolley to the sink areawhere the large chunks of rocks are removed from the boxes by hand and stacked onthe lids (Movie 2 and Movie 3). The residue in the boxes is washed in running waterover two nested screens, a coarse (1-3 mm mesh opening) screen to remove largenodules and rock pieces, and a fine (0.59 mm opening) screen (Movie 4 and Movie 5).Material caught on the coarse screen is returned to the acid, and the residue from thefine screen is spread in trays to dry at room temperature (Figure 11) or under heatlamps. When dry (Figure 12 and Figure 13), it is sorted under the microscope to pick outsmall fossils, mainly bone scrap and teeth. Schiebout's previous experimentation withheavy liquid techniques (sodium polytungstate) for chemical separation of fossils fromscreened residue from Big Bend, Texas, has suggested that visual picking for theserocks is equally efficient.

Spent acid solution (Movie 2) is poured into an 85-gallon waste barrel to awaitneutralization using sodium bicarbonate (Arm and Hammer© brand baking soda) toraise the pH of the waste water to the EPA-required 5.5-7.0 range allowed for disposal.The amount of rock disaggregated for each liter of glacial acetic acid used, variesamong the sites at Fort Polk because of such factors as the nodule size and degree ofcementation of the rock. Kilograms dissolved per liter during the last year are: 1.0 forTVOR Site, 0.6 for Stonehenge Site, and 1.1 for DISC Site. Acid is purchased as glacialacetic in 55-gallon drums. Safety equipment routinely used in the bulk acid lab includesrubber gloves and goggles, and respirators which filter acid fumes and organics, wornduring handling of the glacial acetic acid (Figure 14 and Figure 15).

The fossil specimens are stored in large gelatin capsules, so that individual fossils canbe numbered, studied, and photographed (Figure 16). A pin is inserted through the capend of the capsule and fixed so it cannot slide in or out, by a drop of acetone-solubleglue on the exterior of the pierced end of the capsule. The specimen number is writtenon the portion of the capsule containing the pin, and a tiny fossil is stuck to the pin point

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on a small piece of wax. The use of wax instead of glue allows shifting the fossil’sorientation or removing it if necessary, for example, for SEM photography. In SEMphotography, LEIT-C-PLAST, a plastic conductive carbon cement, is used to mount thefossils so they do not need to be coated.

CARBONATE NODULE CONGLOMERATE FOSSIL SITES

Paleocene, Big Bend National Park, Texas

Carbonate nodule concentrations in Big Bend yielded a significant portion of the faunaof the Paleocene Black Peaks Formation (Schiebout 1974). The Big Bend Paleocenesite, Joe’s Bonebed conglomerate, was a small lens of nodule-rich conglomerate(roughly 1 m across and 8 cm in maximum thickness), that was collected and wetscreened as part of Schiebout’s doctoral research. It was a muddy granuleconglomerate (terminology of Folk 1968, p. 28, 29), rich in clay and lacking carbonatecement (Schiebout 1974). No acid dissolution was necessary for disaggregation.Instead, rock was warmed in an ordinary oven to dry it completely, soaked in the dry-cleaning fluid Varsol, and immersed in water. Water displaced the Varsol and broke therock into its components. After screening, the residue was picked to yield small mammalteeth, including plesiadapoid primates, multituberculates, and insectivores (Schiebout1974). Schiebout (1974, p. 41) concluded that the Joe’s Bonebed deposit was formedby erosion on a floodplain, and did not involve addition of material from the large,through-going rivers of the region that were carrying considerable sand which wouldotherwise have been a more prominent component in the conglomerates.

Cretaceous, Big Bend National Park, Texas

Caliche nodule and limestone pebble conglomerates from the Late Cretaceous(Judithian Age) Upper Shale member, Aguja Formation of Big Bend National Park,Texas were acid processed from 1995-1998 at LSU. Material totaling approximately1,630 kg from five conglomerate horizons, all within 20 m stratigraphically, wascollected and processed, yielding terrestrial and marine vertebrates (Sankey 1998).Previously, these conglomerates were usually ignored by paleontologists as a source offossils because of the difficulty in disaggregating them. Although Lehman (1985) notedconglomerates in many of his stratigraphic sections through the Aguja Formation, noone had attempted to disaggregate and screen them before the current project. Theconglomerates (Figure 17 and Figure 18) are considered to be concentrates from thebases of channels deposited on a marshy coastal plain during the final regression of thewestern Interior Seaway in southern North America. Small fossil teeth and bonesrepresent fish and sharks from the brackish marine environment, and salamanders,lizards, turtles, dinosaurs, and mammals, which, along with the nodules, representerosion and reworking from terrestrial and freshwater habitats. Thirty-six vertebrate taxahave been identified (Sankey 1997; Sankey and Schiebout 1997, Sankey 1998).

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Miocene, Fort Polk, Louisiana

There are seven stratigraphically distinct localities at Fort Polk from which vertebrate-fossil-bearing conglomerate is being processed. The Discovery Site (DISC), is thestratigraphically highest site that will be discussed here. Stonehenge, Gully, and DISCcrop out in ascending order and TVOR S and TVOR are probably stratigraphically lowerthan Stonehenge, although lack of outcrops makes this uncertain. All are from theMiocene upper Fleming Formation, Castor Creek Member, Barstovian Land MammalAge (Schiebout 1994, 1996, 1997a; Schiebout et al. 1996). Three of the Fort Polkconglomerate sites are located in bulldozed, man-made sites and the remaining four increek beds or in erosional gullies. The largest man-made exposure, DISC Site, covers a7.5 acre bulldozed area. The main conglomerate layer exposed at DISC is almost flatlying and underlies at least two acres. It is usually covered with a clay and noduleerosional veneer (Figure 19 and Figure 20), and is seen clearly only in freshly dugsurfaces or areas where erosional gullies cut the layer. In some areas of the site, thereare two distinct layers of the extensive conglomerate separated by an approximately 2cm thick clay layer, and in others, there is a single conglomerate layer (Figure 21). Thedeposit’s topology may well have been that of a perforated sheet. The upper layer,where two conglomerate layers are present, is finer grained and richer in sand, withfewer nodules. Cross bedding can be seen in some places. Isolated small lenses ofconglomerate are also exposed at DISC (Figure 22).

Conglomerate in natural exposures like TVOR Site (Figure 23 and Figure 24) is usuallya near horizontal, tabular ledge, between 10-25 cm thick, and about 1 to 1.5 m wide,often showing some evidence of trough cross-bedding. It is usually overlain andunderlain by a massive gray clay of variable thickness, often with common CaCO3

nodules. Jones et al. (1995) provided a detailed description of rocks and of cores drilledin the Fort Polk Miocene fluvial deposits for the purpose of understanding the processesof deposition that yielded the fossils. For more detailed description of thin sectionpetrography than given below, see Jones et al. (1995) and Schiebout (1997b), and formore detailed site descriptions see Schiebout (1997b). Terminology of the followingdiscussion is after Folk (1959).

Conglomerates at Fort Polk are composed primarily of well-rounded, carbonate peloids,which appear similar to the carbonate nodules found in the paleosols of the cores(Jones et al. 1995). The lower level of the main productive conglomerate at DISC is apelsparite composed of carbonate peloids (51%) and other sedimentary rock fragments(22%), which are cemented with carbonate cement (Figure 25). Some of the carbonatepeloids show septarian structure formed by relatively slower desiccation andcrystallization of their center. The stratigraphically higher layer of the main conglomerate(Figure 26) is a carbonate arenite made up of carbonate peloids, quartz grains, feldspargrains, volcanic rock fragments, and other sedimentary rock fragments and containingless cement. The conglomerate bed from an even more fossil-productive Fort Polk site,TVOR Site, (Figure 27) is a pelsparite composed mainly of carbonate peloids (41%)cemented by either carbonate cement (11%) or a black mineral cement (18%), whichformed before the carbonate cement, probably including iron and manganese oxides.

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Miocene, East Texas

A single boulder of pedogenic-nodule-rich conglomerate (Figure 28) from the FlemingFormation, found at a classic east Texas Miocene site known for yielding mammals ofthe Cold Spring Local Fauna, is yielding microvertebrates after acid dissolution andscreening (Schiebout and Ting in press). This work extends the dissolving ofconglomerates from the Fleming Formation westward into Texas and offers thepossibility of biostratigraphic correlation of faunas of small vertebrates of the Fort Polkregion with a sequence of Miocene faunas developed primarily on large vertebrates(Wilson 1956; Patton 1971; Prothero and Manning 1987).

Recognizing the Rocks in the Field

Surface nodules are certainly a good clue, but not all areas in which nodule-richpaleosols have eroded to litter the surface will have lenses of ancient pedogenic noduleconcentrates. The presence of light-colored calcium carbonate nodules in rocks alsorich in iron oxides and/or manganese gives a fairly distinctive surface texture (Figure 23and Figure 29). In all three areas under study the rocks in question are among the mosterosion resistant of the deposits and at the Cretaceous and Miocene sites, they are byfar the most resistant and are recognizable in small outcrops as seen in Figures 22, 23,29, and Figure 30.

TAPHONOMY

Formation of the Conglomerates

Fossiliferous, nodule-rich conglomerates form in areas where pedogenic nodules andvertebrate remains are a substantial part of the coarse fraction of sediments available;i.e., they are likely to be prominent in fine-grained systems in regions where calcareoussoils tend to form, and where rates of deposition are comparatively slow, allowing gooddevelopment of soils. Kraus (1997) reported better developed paleosols in Eocenealluvial beds of the Willwood Formation in Wyoming in areas estimated to have had a0.3 and 0.4 mm/yr rate of rock accumulation than for areas in which the rate ofaccumulation was estimated between 0.6 and 0.7 mm/yr. Kraus and Bown (1993)estimated the Willwood rates of accumulation. In the late Paleocene, the Big Bend areareceived much slower deposition and is less fossiliferous than late Paleocene sitesfarther north like those of Wyoming, some of which received over 25 times moresediment per million years (Sloan 1987; Schiebout 1995). It is possible that the breakingof bones and teeth, fragments of which are common in the screening residues, hasbeen facilitated by the nodule development itself. Porous bone tends to become heavilyencrusted and/or broken where nodules are forming (Figure 31). This has resulted in agenerally poor paleontological record for areas of slow deposition and nodule-rich soils,in part because of damage to the fossils and in part because of problems involved ingetting the nodular material off of the fossils.

Concentration of nodules and bones can result from reworking during channel

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meandering, overbank fluvial erosion due to flooding, avulsion, or erosion resulting froma drop in base level that causes rivers to entrench, overbank addition to interfluvialareas to slow or stop, and erosion to begin to denude the interfluves. Theconglomerates of very limited lateral extent, like Joe’s Bonebed conglomerate from thePaleocene deposits of Big Bend or small lenses exposed at the DISC Site at Fort Polk(Figure 22), probably represent material washed into small channels or low areas on thefloodplain. The Cretaceous Aguja conglomerates are examples of lag deposits in largerchannels. Wells (1983) discussed pedogenic nodule lag deposits that included somefish and mammal material, from the bases of small channels in sand-poor redbeds fromthe Eocene deposits of northern Pakistan, and that he interpreted as formed by short-lived, briefly active streams, in an environment analogous to parts of modern southernand central Australia.

The most laterally extensive deposit at DISC site in the Louisiana Miocene represents alocally variable blanket of material concentrated from soils by erosion. The thinmudstone layer between upper and lower portions of the main conglomerate mayrepresent an avulsion or crevasse splay event flooding part of the area or mayrepresent local sheet wash. The extreme fragility of some of the fossils recovered fromthe DISC site precludes long transport, but the well-rounded condition of many bonepieces (particularly those from larger animals), suggests some transport for them,perhaps including several cycles of reworking. A larger scale example of similarprocesses has been reported from the Lower Jurassic of South Africa, where regionaldegradation resulting from a base level drop produced a pedogenic nodule concentrate0.5-1.25 m thick covering more than 11,000 square km, rich in vertebrate remains froma large synapsid (mammal-like reptile), and having the topology of a perforated sheet(Smith and Kitching 1997). DISC has also yielded remains of large vertebrates (horsesand the early camel relative Prosynthetoceras, (Figure 32). They were found at orslightly above its upper surface and probably represent animals buried when depositionof overbank fine sediment resumed in the area. Behrensmeyer (1982) discussedpaleosol assemblages representing 4,000-9,000 year durations in her discussion of timeinterval sampling in a terrestrial environment, and Smith and Kitching (1997) estimated50,000 years for the accumulation of the widespread condensed bed they studied in theJurassic of South Africa. The accumulation of the DISC main conglomerate probablytook more time than the development of regular paleosol fossil assemblages, because itincludes a period of denudation and concentration in addition to paleosol formation, andprobably less time than the thicker and more extensive South African deposit.

Bown and Kraus (1981) discussed vertebrate concentrations in mudstones from lowerEocene floodplain sediments of Wyoming, concluding that the bones accumulated aslags in the A horizons of paleosols with little effect of sorting in water. They reported thatsmall mammal teeth make up 70% of the teeth recovered, teeth that would readily travelwith sand-sized particles, and used their presence, the lack of sand, and other evidenceto conclude that hydraulic sorting was not significant in these deposits (Bown and Kraus1981 p. 48-49). All of the Fort Polk conglomerates include nodules and bones roundedin water transport (Figure 25-27, and Figure 33), some show distinctive cross-bedding

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(Figure 24), and some of the most fossiliferous include considerable sand (Figure 27).Many of the fossil remains from the pedogenic nodule conglomerates moved assedimentary particles when carried by water, as did vertebrate remains discussed byVoorhies (1969), Dodson (1973), and Korth (1979). Those that accumulated in channellags, for example, the deposit at TVOR Site, Fort Polk, were certainly subjected to suchprocesses. Small mammal teeth (Figure 34) are moderately transportable in watercompared to other skeletal elements. They belong to Korth’s (1979) dispersal categoryII or III, with I having the lowest settling rate (ribs), and IV the highest, for example,mandibles. High percentages of fish teeth (including pharyngeal teeth) are found withthe terrestrial mammal teeth at Fort Polk (Table 1, Figure 35). When identifiable, theyhave proved to be from fresh-water fish, that had to be either transported into the areaswhere nodules and terrestrial vertebrate teeth were accumulating, or the nodules andmammal teeth had to be transported to join them. These factors indicate that the FortPolk conglomerates include more reworking and transport in water and mixing ofmaterials, and comparatively more time averaging than the fossil accumulationsdescribed by Bown and Kraus (1981). The lower part of DISC main conglomerate ismore similar to deposits such as those described by Bown and Kraus (1981) than areTVOR or Stonehenge sites, where more transport occurred, at Fort Polk. It shows alower concentration of all kinds of teeth, fewer fish teeth per unit of weight whencompared to mammals, and more large bone fragments versus small bone fragments,than they do (Figure 35, Table 1). A rough correspondence between degree ofcementation with calcium carbonate and productivity of teeth, including terrestrialmammal teeth, is seen at Fort Polk sites. Conglomerates like TVOR where morewinnowing has taken place have better cementation (Figure 27) and more fossils,including a higher percentage of fish teeth (Table 1). Clayey conglomerates like DISC(Movie 1, Figure 21) have fewer fossils because they represent both less erosion andconcentration of the harvest of small teeth from soils, and less transport which addedremains such as those of fresh-water fish.

Fort Polk conglomerate sites offer the possibility of studying a progression of sitesthrough time in the Louisiana Miocene. Their fossil faunas are being examined fortrends. Lindsay (1972) reported a size increase upsection in the rodent Copemys in theCalifornia Barstovian, and initial results examining four sites (from lowest to highest:TVOR, Stonehenge, Gully, and DISC Sites), also suggested a size increase in thisanimal through time. Further screening and analysis, however, produced a very differentpicture (Figure 36), emphasizing the value of continuing processing to extract more thanthe handful of small forms initially available.

The unique situation and method of recovery of fossils at Fort Polk guarantees resultsas long as the conglomerates can be located and processed. The faunal list, currentlyincluding 26 land mammals belonging to nine orders, may eventually include 100mammals, like the Myers Farm Site in Nebraska (Corner 1976). Although more than3,000 specimens are catalogued, screening continues because it is especially importantto increase samples of new species and rare forms. Hedgehogs (Schiebout 1996) and anew species of tiny beaver are represented by less than ten teeth apiece. In a Miocene

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possibly warmer than the modern western Louisiana climate, frugivorous bats mighthave survived, but none have been found as yet.

The close association of vertebrate faunas and pedogenic nodules in the conglomeratesoffers an opportunity for fruitful geochemical research. Geochemical studies of delta13Cand delta18O compositions of pedogenic carbonates from the Fort Polk Miocenepaleosols and nodule-rich conglomerates are underway by Paul Aharon (personalcommun., 1997; Schiebout 1997b). Pedogenic nodule delta13C compositions reflectoriginal soil CO2 which, in turn, reflects the nature of the original biomass, and delta18Ois known to have a very strong positive correlation with the isotope composition of localrainfall (Cerling 1984).

CONCLUSIONS

Pedogenic nodule conglomerates, no matter how unprepossessing, are potentialvertebrate fossil sites. The nodule horizons also offer a fossil concentration method thatmay be stratigraphically repeated to offer a periodic sampling. They are recognizable invery small outcrops and are thereby recognizable even in heavily vegetated areas suchas eastern Texas and western Louisiana. Although extraction of large faunas from themis a large-scale, slow endeavor, use of these rocks makes vertebrate paleontologysomewhat convergent upon micropaleontology, where most selected rock samples willbe productive if properly prepared. Vertebrate paleontology in general depends on ahigh degree of luck for preservation and discovery to yield specimens, whereas theseconglomerates offer a fossil concentration method that works with a high degree ofcertainty, and their resistance to erosion makes locating them relatively easy. Areas ofparticular promise for fossils from pedogenic nodule conglomerates include: findingsmall vertebrates at sites where none were known, finding rare vertebrates whereoutcrops are minuscule, stratigraphic repetition of good sites, and finding vertebratesclosely associated with pedogenic nodules on which geochemical work can providedata on ancient soils and vegetation.

ACKNOWLEDGMENTS

Support for research in Big Bend National Park was provided by the National ScienceFoundation, the Dinosaur Society, the LSU Geoscience Associates, the LSU Museumof Natural Science, the LSU Department of Geology and Geophysics, and JoeSchiebout. Many Museum of Natural Science volunteers have helped with the currentresearch on Big Bend. They include Jean Sankey, Jason Ramcharan, Margaret Smith,Charlsa Moore, Tina Aherns, and Sara Wachter. Research in Big Bend National Parkwas conducted under Antiquities Act Permits granted to the LSU Museum ofGeoscience and the LSU Museum of Natural Science. Cooperation of U. S. NationalPark Service employees is much appreciated. I wish to thank Frances and Earl Hodges,landowners near Cold Spring, Texas, for their cooperation in research in that area.

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Help from individuals at Fort Polk, especially Director of Public Works, Rory A.Salimbene, LTC, EN and his environmental staff, including Dr. Charles Stagg, JamesGrafton, and Robert Hays, made the western Louisiana research possible.Paleontological research on Fort Polk is currently supported by U. S. Army FORSCOM,on contracts to the LSU Museum of Natural Science, administered by Dr. FrankServello, Fort Worth District, U. S. Army Corps of Engineers, issued to Prewitt andAssociates, Inc. Mapping in the vicinity has also been supported by the United StatesGeological Survey via the EDMAP program. Support has also been provided by theLSU Museum of Natural Science. Pam Borne and Megan Jones helped with ColdSpring field work and they and Alton ("Butch") Dooley, Jr., Brett Dooley, and many otherLSU students and faculty assisted in field work on Fort Polk. Julitta Kirkova did thinsection point counts. The LSU Museum of Natural Science vertebrate paleontologicalbulk acid lab is located in the LSU Basin Research Institute Well Core and SampleLibrary, and their cooperation is appreciated.

Timothy Patterson from Carleton University in Ottawa, Canada, two anonymousreviewers, Louis Jacobs from Southern Methodist University in Texas and Ruth Hubert,Arnold Bouma, Paul Aharon, and Paul Heinrich from LSU provided helpful comments onthis manuscript. Photographs are by Judith Schiebout unless otherwise noted. Thefollowing staff of the LSU Center for Instructional Technology provided assistance:Christie Cazes digitized and improved images for the figures and table, Tom Davis shotvideotape, Dan Brock edited and digitized it, and Marti Ratcliff and Dan Brock did thefinal editing.

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Sankey, J. T. 1998. Vertebrate paleontology and magnetostratigraphy of the Upper AgujaFormation (Late Campanian), Talley Mountain area, Big Bend National Park, Texas. UnpublishedPh.D. dissertation, Louisiana State University, Baton Rouge, Louisiana, USA.

Schiebout, J. A. 1974. Vertebrate paleontology and paleoecology of Paleocene Black Peaks Formation,

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Big Bend National Park, Texas. Texas Memorial Museum Bulletin, 24:1-87.

Schiebout, J. A. 1979. Paleoecology of the Paleocene Black Peaks Formation, Big Bend National Park,Texas. Proceedings of the First Conference on Scientific Research in the National Parks. NationalPark Service Transactions and Proceedings, 5:737-742.

Schiebout, J. A. 1981. The stratigraphic and paleogeographic importance of Paleocene and early Eocenedeposits in Big Bend National Park. Proceedings of the Second Conference on Scientific Research inthe National Parks, National Park Service Transactions and Proceedings, 5:332-347.

Schiebout, J. A., Rigsby, C. A., Rapp, C. D., Hartnell, J. A. and Standhardt, B. R. 1987. Stratigraphy ofLate Cretaceous, Paleocene, and early Eocene rocks of Big Bend National Park, Texas. Journal ofGeology, 95:359-375.

Schiebout, J. A. 1994. Fossil vertebrates from the Castor Creek Member, Fleming Formation, westernLouisiana. Gulf Coast Association of Geological Societies Transactions, 44: 675-680.

Schiebout, J. A. 1995. The Paleocene/Eocene transition on Tornillo Flat in Big Bend National Park,Texas. In V. L. Santucci and L. McClelland (eds.), National Park Service Paleontological Research,National Park Service Technical Report NPS/NRPO/ NRTR-95/16: 40-45.

Schiebout, J. A. 1996. A Miocene hedgehog (Mammalia: Erinaceidae) from Fort Polk in westernLouisiana. Occasional Papers LSU Museum Natural Science, 70: 1-9.

Schiebout, J. A., Jones, M. H., Wrenn, J. H. and Aharon, P. R. 1996. Age of the Fort Polk Mioceneterrestrial vertebrate fossil sites. Gulf Coast Association of Geological Societies Transactions,46:373-378.

Schiebout, J. A. 1997a. The Fort Polk Miocene microvertebrate sites compared to those from east Texas.The Texas Journal of Science, 49:23-32.

Schiebout, J. A. 1997b. Paleofaunal survey, collecting, processing, and documentation at twolocations on Fort Polk, Louisiana. Louisana State University Report to the Corps of Engineers, FortWorth District, Texas, USA.

Schiebout, J. A. and S. Ting. in press. Miocene terrestrial microvertebrates recovered fromconglomerate rich in pedogenic nodules, Fleming Formation near Coldspring, Texas. TexasJournal of Science, v. 50(3).

Judith A. Schiebout, Julia T. Sankey, Barbara R. Standhardt, and Jason Ramcharan. in press. LouisianaState University Museum of Natural Science Collections from Late Cretaceous through EarlyEocene Microvertebrate Sites, Big Bend National Park, Texas. National Park Service PaleontologicalResearch, National Park Service Technical Reports.

Sloan, R. E. l987. Paleocene and latest Cretaceous mammal ages, biozones, magnetozones, rates ofsedimentation, and evolution. Geological Society of America Special Paper, 1:209.

Smith, R. and Kitching, J. 1997. Sedimentology and vertebrate taphonomy of the Tritylodon Acme Zone;a reworked palaeosol in the Lower Jurassic Elliot Formation, Karoo Supergroup, South Africa.Palaeogeography, Palaeoclimatology, Palaeoecology, 131:29-50.

Voorhies, M. R. 1969. Taphonomy and population dynamics of an early Pliocene vertebrate fauna, KnoxCounty, Nebraska. University of Wyoming Contributions in Geology Special Papers, 1: 1-69.

Wells, N. A. 1983. Transient streams in sand-poor redbeds: early-Middle Eocene Kuldana Formation ofnorthern Pakistan. Special Publications International Association Sedimentologists, 6:393-403.

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Wilson, J. A. 1956. Miocene formations and vertebrate biostratigraphic units, Texas Coastal Plain.American Association of Petroleum Geologists Bulletin, 40:2233-2246.

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Figure 1. Shrew upper molar from TVOR Site, Miocene of Fort Polk,Louisiana. Occlusal view. Scanning electron micrograph by XiaogangXie and Suyin Ting.

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Figure 2. Location map showing Fort Polk in Louisiana, and Big Bend National Parkand Coldspring in Texas. Permits are required for research on Fort Polk and in BigBend National Park.

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Figure 3. Lateral view of merychippine horse mandible encased in pedogenicnodular material from DISC Site, Miocene of Fort Polk, Louisiana.Photograph by Kerry Lyle.

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Figure 4. Joe’s Bonebed, Paleocene Black Peaks Formation, Big Bend NationalPark, Texas. Fluvial mudstone slopes are heavily covered with pedogenic nodules.Remains of large and medium-sized vertebrates were recovered from float all alongthe slope pictured here. A small lens of nodule conglomerate from this slope yieldedthe microvertebrate fauna from Joe’s. Suyin Ting (L), Joe Schiebout (M), and JillHartnell (R).

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Figure 5. Judy’s conglomerate, Late Cretaceous, Aguja Formation, Big Bend NationalPark, Texas, with Julia Sankey. Photograph by Jean Sankey.

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Figure 6. LSU field crew wet screening untreated mudstone in the RioGrande, Big Bend National Park, Texas.

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Figure 7. Robert Rainey (L) and Judith Schiebout (R) bag conglomerate at Joe’sBonebed, Big Bend National Park, in 1970. Photograph by John A. Wilson.

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Figure 8. Helicopter carries bags for screening from remote sites inBig Bend National Park, Texas in 1984.

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Figure 9. Bags of rock from the Louisiana Miocene as brought in fromthe field, closed with duct tape. Airless jackhammer and conglomeratepiece from Stonehenge Site rest on the bags.

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Figure 10. Slabs of conglomerate from Stonehenge Site, Mioceneof Fort Polk, which have been removed by heavy equipment froma gully where they were exposed. Later, they were broken up bygasoline-powered jackhammer for bagging. Robert Hays (L) andSuyin Ting (R). Photograph by Pam Borne.

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Figure 11. Dumping a screen on a drying tray.

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Figure 12. Dried screening residue from Stonehenge Site showingnodules darkened with iron and manganese oxides. Dr. Schieboutindicates a bone fragment.

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Figure 13. Dried screening residue from DISC Site showing noduleslarger and lighter colored than those from Stonehenge Site.

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Figure 14. Dr. Ting in goggles and respirator, facial protective gearworn during handling of the glacial acetic acid.

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Figure 15. Dr. Ting and Casey Foote in protective gear are preparingto pump acid from the 55-gallon barrel for dilution to 10% and additionto the plastic boxes of rock. Rock saw in background is not used inthis project.

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Figure 16. First lower molars of the rodent Copemys, illustrating mounting andnumbering method used for microvertebrate fossils from the Miocene of Fort Polk.Gray clay holds the row of capsules in place.

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Figure 17. Judy’s conglomerate, cross sectional view, Late Cretaceous, AgujaFormation, Big Bend National Park, Texas. Nodules and bone pieces show as whiteflecks. Photo by Julia Sankey.

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Figure 18. Dry (L) and wet (R) pieces of Julia’s conglomerate from the LateCretaceous, Aguja Formation, Big Bend National Park. Photo by Julia Sankey.

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Figure 19. Weathered, man-made surface of the main conglomerate at DISC Site,Miocene of Fort Polk. It will be broken up with sledge or airless jackhammer andshoveled into bags.

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Figure 20. Incisor on weathered surface of the main DISC conglomerate.

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Figure 21. Cleaned cross sectional surface showing contact of main conglomeratefrom DISC site and underlying overbank mudstone.

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Figure 22. Erosion at DISC Site revealing several minor layers of pedogenic noduleconglomerate.

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Figure 23. TVOR Site conglomerate, Miocene of Fort Polk, being photographed bygeoarchaeologist Timothy Dalbey of the Corps of Engineers, Fort Worth District Thisis a color version of Figure 2 of Jones et al. (1996) and Photograph 8 of Schiebout(1997b). Site looks very similar to its appearance when first discovered.

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Figure 24. Close up, cross sectional view, of the conglomerate at TVOR Site showingboth fresh dark surfaces which have been broken and lighter colored weatheredsurface. Cross bedding is evident.

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Figure 25. Thin section from lower part of DISC main conglomerate under polarizedlight. "A" indicates a nodule showing septarian cracks and "B" is a partially dissolvednodule. Scale = 1 mm. This is a color version of Photograph 13 (Schiebout 1997b).Photograph by Julitta Kirkova.

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Figure 26. Thin section under polarized light, cut a few cm higher than Figure 25,from the upper part of the main conglomerate at DISC Site, showing more quartzsand. Scale = 1 mm. Photograph by Julitta Kirkova.

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Figure 27. TVOR Site conglomerate under polarized light, showing rounded nodulesand sand in a calcite cement. "A" indicates a nodule and "B" is a quartz grain. Scale =1 mm. This is a color version of Photograph 12 (Schiebout 1997b). Photograph byJulitta Kirkova.

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Figure 28. Sawed, cross sectional surface of a conglomerate boulder from a sitenear Coldspring, Texas at which mammals of the Miocene Cold Spring Local Faunahad been recovered.

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Figure 29. Erosional gully, TVOR S Site in the Miocene of Fort Polk,shows a pedogenic nodule conglomerate, indicated by an arrow. Thissite is a kilometer from TVOR. Photo by Megan Jones.

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Figure 30. Close up of conglomerate seen in Figure 29. Photo by Megan Jones.

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Figure 31. Nodule encrusted and cracked large mammal bone in place at Joe’sBonebed in mudstone of the Paleocene Black Peaks Formation in Big Bend NationalPark, Texas.

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Figure 32. Mandible of Prosynthetoceras francisi exposed by weathering of a man-made surface at DISC Site. Displacement of the piece of the mandible from the restmay have resulted from the passage of heavy machinery over the site. An arrowindicates the displaced piece.

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Figure 33. All fossils from a preliminary picking of the main conglomerate at DISCSite, Miocene of Fort Polk. Teeth and identifiable bone will be removed in a secondpicking. This is a color version of Photograph 16 (Schiebout 1997b).

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Figure 34. All mammal teeth from a session of picking at TVOR Site. A geomyoidrodent tooth is indicated by the arrow.

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Figure 35. 300-counts of kinds of fossils from TVOR Site (A, B, C)and DISC (D). Figure 35C is modified from figure 8 and 35D ismodified from figure 7 of Schiebout (1997b).

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Figure 36. Bivariate plot of length versus width of the first lower molars of the rodentCopemys from sites on Fort Polk, Louisiana and from Coldspring, Texas. Three sites onFort Polk, Stonehenge, Gully, and DISC, crop out in ascending stratigraphic order, andTVOR is probably stratigraphically lower than Stonehenge, although lack of outcropsmakes this uncertain.

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Table 1. Fish teeth and mammal teeth per 100 kg. of rock processed at three sites onFort Polk.

TVOR Stonehenge DISC

Fish/100 kg. 1,396.7 550.0 42.8

Mammal/100 kg. 86.7 62.8 8.6

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Movie 1. Rock from Discovery Site in almost spent acid solution. Apiece is extracted and rinsed to show the undissolved rock. DiscoverySite rocks are high in clay compared to those from other Fort PolkMiocene sites. File size: 801 KB.

Movie 2. Spent acid is bailed from the plastic boxes for removal to an85-gallon barrel for neutralization, pH testing, and aeration beforedisgard. Boxes are then moved to a gurney for transport to thescreening area. File size 1.1 MB.

Movie 3. Boxes containing rocks from Stonehenge Site, Fort Polk,Louisiana, which have spent a rotation in acid, are brought by gurneyto the washing sink. Rocks are rinsed and set aside on the box lids.Rocks will be returned to the emptied boxes after screening of theresidue, and the boxes returned to their spots in the ranks, for furtherdissolution. File size 867 KB.

Movie 4. The small amount of spent acid left in the boxes is bailedinto the screens, and residue in the boxes is emptied into the screens.Most of the spent acid was previously removed by bailing before theboxes were transported and was carried to the waste barrel. Spentacid in the boxes was tested for pH at the sink, using pH strips whichchange color to indicate acid levels. File size 926 KB.

Movie 5. Washing of residue from the boxes on the nested screens.File size 471 KB.

Movies of the LSU Museum of Natural Science bulk acid lab feature Dr. Suyin Ting,head of the lab, and student assistant Casey Foote. Lab procedures are illustratedusing material from the Miocene of Fort Polk, Louisiana. Film was shot during aregular screening session on August 5, 1997.